Mobile system for seismic emission with fixed coupling devices, and method therefor

ABSTRACT

Seismic emission system for use in an underground formation, using at least one seismic source movable successively in a plurality of prepared activation locations and method for implementing same. 
     The system comprises for example a plurality of coupling devices ( 1 ) including each for example two anchor elements ( 12, 9 ′) tightly coupled with the formation (by cementing for example). These anchor elements are secured to the seismic source (V) such as a vibrator of any type or an impulsive source by transmission elements ( 10, 13 ). The seismic source is coupled with coupling device ( 1 ) by fast locking means ( 18, 19 ). Coupling devices ( 1 ) can be preinstalled at a plurality of prepared activation locations above the formation to be monitored. Seismic monitoring operations can be carried out by moving at least one seismic source successively between the different locations where coupling devices ( 1 ) are installed.

FIELD OF THE INVENTION

The present invention relates to a seismic emission system for use in anunderground formation using a source movable successively between aplurality of prepared activation locations and a method for implementingthe system.

Such a seismic emission system finds applications notably in the fieldof seismic operations where seismic images of an underground formationto be explored are formed from elastic waves picked up by suitableseismic receivers, these waves being reflected by the subsoildiscontinuities in response to waves transmitted by a source such as anelectromechanical vibrator.

The system according to the invention is more particularly useful in thecontext of long-term monitoring operations in an underground reservoirunder development (a fluid storage reservoir for example or an oilreservoir), referred to as repetitive seismic surveys, where seismicimages of the subsoil obtained at regular intervals are compared so asto detect changes that may have occurred therein as a result of itsdevelopment. These are long-lasting operations because the variations tobe observed are relatively slow.

BACKGROUND OF THE INVENTION

Seismic monitoring of an underground reservoir is generally carried outby coupling with the ground seismic sources and receivers in variouscombinations where the sources and/or the receivers are arranged at thesurface or in the vicinity thereof, or in one or more wells through theexplored formation. Series of seismic emission-reception cycles arecarried out by changing each time the location of the seismic source inrelation to the axis of the well where the receivers are installed,according to a technique referred to as walk-away technique, and byrecording the arrivals at receivers R1 to Rn as a function of thepropagation time t.

The seismic sources used are in most cases electromechanical vibrators:electrohydraulic, piezoelectric vibrators, etc. Piezoelectric typevibrators are for example described in patent FR-2,791,180 or in U.S.Pat. No. 5,360,951.

Monitoring of the evolution of reservoirs generally requires seismicmonitoring operations at spaced-out intervals. In practice, the surfaceseismic equipment has to be reinstalled for each new seismicrecording-session and the conditions of emission of the previous seismicoperations are preferably reproduced.

A known method of monitoring a hydrocarbon reservoir or an undergroundfluid reservoir comprises using a monitoring system including receivingantennas formed by interconnecting seismic receivers, permanentlyinstalled respectively in shallow holes, with connection means to whichcables linked to a seismic laboratory can be connected, and a vibratortruck that is moved in the field.

Using a mobile source such as a vibrator involves drawbacks, especiallywithin the context of periodic monitoring of an underground storagereservoir. A movable source does not allow a sufficient reproducibilityin time of the seismic waves emitted. It is very difficult to positionthe source exactly at the same points it occupied during the previousemission-reception cycles and, in the event that this point is exactlythe same, to obtain exactly the same ground coupling coefficient. Thisrepresents a constraint for the (generally overloaded) site operator whohas to facilitate its evolution, all the more so if the source used hasto be triggered substantially in the same places as during the previousemission-reception cycles.

Patent FR-2,728,973 (U.S. Pat. No. 5,724,311) describes a method and adevice intended for permanent seismic monitoring of an undergroundformation. In the context of regular long-term monitoring operations inan underground zone, a seismic emission-reception device is permanentlyinstalled on the development site so as to find each time stableoperating conditions again: identical emission-reception locations,identical quality of coupling with the formation, etc. The devicecomprises a plurality of seismic sources (electromechanical vibratorsfor example) at fixed locations at the surface or buried at a shallowlevel, which are supplied and triggered by a central control andrecording station. The seismic sources and the connection network can beburied or permanently installed at the surface, and associated with atleast one array of receivers permanently coupled with the ground at thesurface or with the wall of at least one well drilled through theunderground zone. All these permanently installed sources whose couplingwith the surrounding formations remains stable and this supply network,at least partly buried, whose surface coverage area is limited, allow tocarry out a series of seismic monitoring operations under stableoperating conditions, without any risk of incompatibility with thedevelopment site activities.

Patent FR-2,728,973 (U.S. Pat. No. 5,724,311) describes another seismicmonitoring device intended for permanent monitoring of an undergroundformation by means of one or more emission-reception sets comprisingeach a source such as a vibrator and a receiving antenna consisting of aplurality of elastic wave receivers such as geophones and/or hydrophoneslowered in a well and coupled with the formation. The seismic source isarranged in a cavity close to the location of the antenna or formed bywidening the section of this well in its upper part. The receivers andthe source are connected to an outside control and signal acquisitionstation. The operations allowing setting of these devices are relativelysimple and the ground coverage area in the various wells is reduced,which facilitates their integration in reservoir development sites.

By means of these permanently installed sources easy to integrate inreservoir development sites or fluid storage sites, and whose couplingquality with the surrounding formations is known and stable, a series ofseismic monitoring operations can be carried out under similar operatingconditions. The seismic trace sets can be usefully compared and theirdifferences reflect the changes occurred in the formations, and notdisturbances linked with climatic variations for example.

However, monitoring operations require permanent coupling of an oftenlarge number of seismic sources, which makes the monitoring installationrelatively expensive.

SUMMARY OF THE INVENTION

The seismic emission system according to the invention allows emissionof seismic waves in an underground formation at a plurality of preparedactivation locations, under excellent reproducibility conditions.

It is characterized in that it comprises, at each location, a couplingdevice including at least one anchor element tightly coupled with theformation at a certain depth and at least one transmission elementsecured to the anchor element, and at least one seismic source movablesuccessively between these various locations and provided with fastlocking means for connection to the transmission element.

According to a first embodiment, the device intended for coupling withthe formation comprises for example at least two anchor elements buriedin the ground (at different depths for example), each one beingconnected by the transmission elements to the seismic source located atthe surface or in the vicinity thereof.

The coupling device comprises for example a first transmission elementassociated with the anchor element which is the closest to the surface,and a second transmission element, the first transmission elementsurrounding the second transmission element and being associated at afirst end with the second anchor element having the greatest depth ofburial, the seismic source being tightly connected to the twotransmission elements during operation.

The first transmission element is tubular for example, the seismicsource resting on its opposite end by means of a seat.

According to a second embodiment, the device intended for coupling withthe formation comprises an anchor element buried at a certain depth, theseismic source being coupled during operation with the transmissionelement secured to this anchor element, and resting on the grounddirectly or by means of support elements.

The system can comprise at least one casing insulating the transmissionelements from the formation.

The locking elements are for example accessible in a pit provided in theground at each location, in which the seismic source is placed duringoperation, the bearing surface on which the seismic source rests beingeither inside or outside the pit.

According to an embodiment, the seismic source is a vibrator ofpiezoelectric, electromagnetic, electrohydraulic or magnetostrictivetype.

If the seismic source is a piezoelectric vibrator, it comprises forexample a motor between a plate resting on the ground and an oppositeplate suited to be fastened to the transmission element duringoperation.

The seismic source can be included in a vehicle provided with operatingmeans for setting and coupling it, or it can be transported by anindependent vehicle.

According to an embodiment, the system also comprises at least oneseismic source permanently installed at a location equipped with acoupling device.

The method for seismic prospecting or monitoring of an undergroundformation according to the invention comprises installing an array ofseismic receivers and coupling them with the formation. It ischaracterized in that it comprises:

installing in the field, in a plurality of predetermined locations, atleast one coupling device comprising at least one anchor element tightlycoupled with the formation at a certain depth and at least onetransmission element secured to the anchor element,

moving at least one seismic source successively between these variouslocations and securing it to the transmission elements of thecorresponding coupling device, and

at each of said locations, triggering the seismic source and acquisitionof the seismic data picked up by the seismic receivers in response tothe seismic signals applied to the formation by the seismic source.

The method can also comprise installing at least one seismic sourcepermanently at one of said predetermined locations equipped with acoupling device.

Permanent installation of the emission system initially amounts tooperations of anchoring the coupling devices at the various emissionlocations considered. With a single easily movable seismic vibratorwhose coupling with the formation at the successive locations is stableand reproducible on account of the anchor devices previously installed,it is possible to carry out complete seismic operations whereas before,as many vibrators as there are emission points were required: the systemtherefore constitutes an economically profitable alternative toconventional devices.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the emission system and of the methodaccording to the invention will be clear from reading the descriptionhereafter of embodiments given by way of non limitative example, withreference to the accompanying drawings wherein:

FIG. 1 diagrammatically shows a first embodiment of the coupling device,

FIG. 2 diagrammatically shows a second embodiment of the couplingdevice, and

FIG. 3 diagrammatically shows an example of implementation of theemission system with anchoring of a vibrator movable successivelybetween the prepared anchoring locations.

DETAILED DESCRIPTION

The emission system comprises an array of coupling devices 1preinstalled in the field at selected locations suited for monitoring ofthe underground formation.

According to the embodiment of FIG. 1, each coupling device 1essentially comprises a transmission element 2 such as a rod or tendon,secured to a coupling element or horn 3 anchored in the ground, andprovided with a locking head 4 allowing fast connection to a seismicsource such as a vibrator V for example. Horn 3 is coupled with theformation at a certain depth, preferably below the weathered layer (WZ).A cavity 5 is created, into which the horn associated with its rod 2 islowered, and horn 3 is cemented so as to couple it with the surroundingformation. Cavity 5 is connected to a pit 6 dug at the surface by a wellprovided with a casing 7. Vibrator V is lowered into pit 6 and connectedto locking head 4 of rod 2. Vibrator V has a second supporting point onthe ground in form of a fixed coupling baseplate 9 such as an annularplate arranged around pit 6.

Seismic vibrator V can be transported from one anchoring location to thenext by an independent vehicle and placed in pit 6 in order to becoupled with device 1. It can also be integrated in a vehicle 8 providedwith operating means (not shown) allowing to shift from an uppertransport position to a lower working position. In this case, thevehicle rests on coupling baseplate 9. Vibrator V is then brought intothe lower position and connected to locking head 4 of rod 2 of couplingdevice 1.

According to the embodiment of FIG. 2, the device comprises two couplingelements such as plates 9′, 12 buried at different depths (preferablybelow the weathered layer WZ). The seismic source rests by means of acoupling collar 11 on the upper part of a tubular transmission element10 (referred to as tube hereafter). At the base thereof, it is fastenedto upper coupling plate 12. Another transmission element 13 such as arod or a cable, for example, secured to lower coupling plate 9 at afirst end and connected by means of a fast locking head 18 to seismicsource V at its opposite end, runs through tube 10. Seismic source Vthus transmits the waves only by means of the two buried plates 9′, 12(and possibly of tube 10 and cable 13 if they are in contact with theformation). Tube 10 and cable 13 can also be uncoupled from the adjacentformations by a casing 20. The wellbore also opens into a pit 6 servingas a protection for fast locking head 18 and vibrator V.

Vibrator V can be of piezoelectric, electromagnetic, electrohydraulic ormagnetostrictive type. A piezoelectric type vibrator comprises forexample (FIG. 2) two plates 14, 15 between which several cylindricalpiles 16 obtained by piling piezoelectric discs are inserted, orpossibly a single pile made by piling annular piezoelectric elements.Lower plate 15 is provided with a tubular extension 17 suited to restagainst collar 11 ending tube 10 in the upper part thereof. Plates 14,15 are provided with a central opening allowing free passage of rod orcable 13. A fast connection means 19 of a well-known type is used tosecure rod 13 (via locking head 18) to upper plate 14 of vibrator V.

Implementation

The device intended for seismic monitoring of an underground formationcomprises (FIG. 3) an array of anchor devices 1 such as, for example,those described above, distributed in the field, and at least one arrayof seismic receivers R permanently installed in contact with themonitored zone. This array of receivers R can be installed in a well W,whether an exploration well or a development well for the monitoredzone, outside a production tubing as described in patent FR-2,674,029(U.S. Pat. No. 5,243,562) so as to keep it clear for other activities;it can also be buried in the ground near to the surface. The array ofreceivers R is connected to a central control and recording station SLby means of connection cables C.

Vibrator V is successively transported to the various locations ofcoupling devices 1, lowered into pit 6 and fastened to the anchorsystems so as to be coupled with the formation by means of elements 3,9, 9′ and 12 described above. Once vibrator V secured to an anchordevice 1, the seismic operations can be started by triggering thevibrator and recording the seismic signals received by receivers R inresponse to the waves emitted. It is then separated from the couplingdevice and transported to the location of another coupling device and soon.

Operations are facilitated by the presence of a vehicle such as thevehicle shown in FIG. 1, provided with operating elements for fasteningvibrator V (integrated in the vehicle in this case) to the transmissionelements in each pit 6.

Embodiments where the or each mobile seismic source used is a vibratorhave been described. It is however clear that the vibrator can bereplaced by any other type of seismic source, an impulsive source forexample.

A method where the formation monitoring operations are carried out bymeans of one or more mobile sources that can be anchored at variouslocations provided with an anchor device 1 has also been described. Itis however possible, without departing from the scope of the invention,to combine this movable emission system with a fixed emission devicewhere one or more seismic sources are permanently installed at a certainnumber of locations equipped with an anchor device 1 as described above.

1. A seismic exploration or monitoring system intended for periodicseismic monitoring of an underground formation, comprising seismicsignal emission means anchorable in the formation by fastening anchormeans tightly coupled with the formation at a certain depth andreception means for signals coming from the formation in response to theseismic signals emitted, characterized in that the anchor means comprisea plurality of coupling devices, each one being preinstalled at alocation of a series of locations selected above the undergroundformation and the emission means comprise at least one mobile seismicsource movable successively between these various locations and providedwith locking means for securing the seismic source successively to thevarious coupling devices, wherein each coupling device comprises atleast a first and a second anchor element buried in the ground atdifferent depths each one being connected by transmission elements tothe seismic source arranged at the surface or in the vicinity thereof, afirst of the transmission elements being associated with the anchorelement which is the closest to the surface, and a second of thetransmission elements being associated at a first end with the anchorelement having the greatest depth of burial, the first of thetransmission elements surrounding the second of the transmissionelements, and the seismic source being tightly connected, duringoperation, to the transmission elements.
 2. A system as claimed in claim1, characterized in that the first transmission element is tubular, theseismic source resting on its opposite end by means of a seat.
 3. Asystem as claimed in claim 1, characterized in that the seismic sourcerests on the ground directly or by means of support elements.
 4. Asystem as claimed in claim 1, characterized in that the transmissionelements are insulated from the formation by at least one casing.
 5. Asystem as claimed in claim 1, characterized in that the locking meansare accessible in a pit provided in the ground at each location whereinthe seismic source is placed during operation.
 6. A system as claimed inclaim 5, characterized in that the bearing surface on which the seismicsource rests is inside or outside the pit.
 7. A system as claimed inclaim 1, characterized in that the seismic source is a vibrator ofpiezoelectric, electromagnetic, electrohydraulic or magnetostrictivetype.
 8. A system as claimed in claim 1, characterized in that seismicsource is a vibrator comprising a motor between a plate resting on theground and an opposite plate suited to be secured to a transmissionelement during operation.
 9. A system as claimed in claim 1,characterized in that the seismic source is integrated in a vehicleprovided with operating means for setting and coupling the source.
 10. Asystem as claimed in claim 1, characterized in that the seismic sourceis transported by an independent vehicle.
 11. A system as claimed inclaim 1, characterized in that it comprises at least one seismic sourcepermanently installed at a location equipped with a coupling device. 12.A method intended for periodic seismic exploration or monitoring of anunderground formation, comprising installing an array of seismicreceivers and coupling thereof with the formation, and anchoring aseismic source with the formation by fastening to anchor means tightlycoupled with the formation at a certain depth, characterized in that itcomprises: installing in the field, at each location of a plurality ofpredetermined locations above the underground formation, at least onecoupling device comprising at least one anchor element tightly coupledwith the formation at a certain depth and at least one transmissionelement secured to the anchor element, moving at least one seismicsource successively between these various locations and securing it tothe transmission elements of the corresponding coupling device, and ateach of said locations, triggering the seismic source and acquisition ofthe seismic data picked up by the seismic receivers in response to theseismic signals applied to the formation by seismic source.
 13. A methodas claimed in claim 12, characterized in that it comprises integratingsaid seismic source in a vehicle provided with operating means forsetting, coupling and moving the source.
 14. A method as claimed inclaim 12, characterized in that it comprises moving the seismic sourcebetween the successive locations, taking the seismic source out of thevehicle and coupling it to the connection element of each couplingdevice.
 15. A method as claimed in claim 12, characterized in that itcomprises installing at least one seismic source permanently at one ofsaid predetermined locations equipped with a coupling device.